Free volume evolution in the process of epoxy curing and its effect on mechanical properties

We use molecular dynamics simulations to study the evolution of free volume during the process of curing of two thermoset epoxy polymers and correlate its volume fraction with the resulting thermo-mechanical properties. The simulations reveal that while the overall volume of the polymer shrinks during cure, the amount of free volume increases. Interestingly, this increase is due to the appearance of large voids (connected regions of free volume) while the number of small and medium-sized voids decreases during cure. We also find correlations between the location of the voids and the local chemical structure of the polymer. An analysis of the individual nanovoids shows a power-law scaling for over three orders of magnitude in volume (0.1 angstrom(3) to 100 angstrom(3)) and a fractal scaling in surface with a fractal dimension similar to 2.37. The effect of free volume on mechanical properties is also quantified. A very small reduction in the free volume fraction (similar to 1%) results in considerable (> 10%) stiffening and strengthening. (C) 2016 Elsevier Ltd. All rights reserved.